Process for the removal of arsenic and chromium from water

ABSTRACT

The present invention provides low cost and highly effective method for the removal of arsenic and Cr(III&amp;VI) from contaminated water using zinc peroxide nanoparticles (20±5 nm) capped with glycerol/PVP/TEA up to the permissible range of drinking water. As Arsenic and chromium occurs naturally in the earth&#39;s crust. When rocks, minerals, and soil erode, they release arsenic and chromium into groundwater. Arsenic and chromium occurs naturally in varying amounts in groundwater in various parts of country from ppb level to ppm level. The average concentration of arsenic and chromium as per USEPA standard in drinking water it is 10 parts per billion and 0.05 ppm (50 ppb) respectively. In drinking water the level of chromium is usually low as well, but contaminated water may contain the dangerous Cr(III&amp;VI). Although Cr(III) is an essential nutrient for humans and shortages may cause heart problems, disruptions of metabolisms and diabetes. But the uptake of too much Cr(III) can cause health effects as well, for instance skin rashes. Cr(VI) is known to cause various health effects Skin rashes, upset stomachs, respiratory problems, weakened immune systems, kidney and liver damage and lung cancer The persons who are drinking water having up to 50 ppb of arsenic and 0.05 ppm chromium over for many years could experience skin damage or problems with their circulatory system, and may have an increased risk of getting cancer. Keeping the above facts we developed a cost effective nanoparticles for the removal of Arsenic and Cr(III&amp;VI) from potable water up to potable range.

FIELD OF THE INVENTION

The present invention relates to a process for the removal of arsenicand Chromium [Cr(III) & Cr(VI)] from contaminated water. Particularly,the present invention relates to a process for the removal of arsenicand chromium from contaminated water using nanoparticles of zincperoxide (ZnO₂) capped with glycerol/triethylamine (TEA)/poly vinylpyrrolidone (PVP). More particularly, the present invention relates to aprocess for the removal of arsenic and chromium from the contaminatedwater to a level that is suitable for safe drinking.

BACKGROUND OF THE INVENTION

Presence of arsenic (As) and chromium metal ions in drinking water hasbecome the issue of global concern. Long-term exposure to even lowconcentrations of arsenic in the drinking water may cause skin, lung orprostrate cancer and cardiovascular, pulmonary, immunological andneurological disorder [Environment Health Criteria 224, Arsenic andArsenic Compounds, Second edition, World Health Organization 2001; S.Shevade and R. G. Ford. Use of synthetic zeolites for arsenate removalfrom pollutant water. Water Res. 38(14-15), 3197 (2004)].

At present there is no effective medicine available. which can treatdisease, causes by arsenic and chromium, so use of arsenic and chromiumfree water can help the affected person to get rid of the symptoms ofarsenic and chromium toxicity. Hence, the requirement of arsenic andchromium free water is urgently desired to mitigate arsenic and chromiumtoxicity and protection of the health of human beings living in theareas affected by arsenic and chromium contamination.

World Health Organization (WHO) & Environment Protection Agency (EPA)has established a level of 10 g/L arsenic and 50 g/L of chromium indrinking water from January 2006. But in several countries likeBangladesh, India, arsenic and chromium concentration in the drinkingwater can be as high as 500 μg/L or more. The reduction of arsenic andchromium from such high concentrations and made it potable as per WHOprescribe limit is a very challenging task.

Arsenic occurs in rocks, soil, water and air in −3, 0, +3 and +5 valencestate. It is widely distributed having average concentration of 2 mg/kg.The burning of fossil fuels, refining of petroleum mining, smelting ofmetals like Zn, Cu, Ni, and Pb, are major anthropogenic sources forarsenic contamination in air, water and soil.

Therefore high toxicity and widespread occurrence created a pressingneed for effective monitoring, measurement and remediation of arsenic insoil and groundwater. The effect and degree of toxicity of arsenicdepends on its inorganic or organic forms and oxidation state. Inorganicarsenicals are more toxic than organic arsenicals and in inorganicarsenicals trivalent form is more toxic than the pentavalent form.

Reference may be made to Y. Lee, I. H. Um and J. Yoon, Arsenic (III)oxidation by iron(VI) (ferrate) and subsequent removal of arsenic (V) byiron (III) coagulation. Environ. Sci. Technol. 37(24), 5750 (2003); B.Daus. R. Wennrich and H. Weiss, Sorption materials for arsenic removalfrom water: a comparative study. Water Res. 38(12), 2948 (2004); S.Bang, G. P. Korfiatis and X. Meng, Removal of arsenic from water byzero-valent iron. J. Hazard Mater. 121(1-3), 61 (2005); Y. S. Shen.Study of arsenic removal from drinking water. J. American Water WorksAssociation, 65(8), 543 (1973) and A. Joshi and M. Chaudhary. Removal ofarsenic from groundwater by iron-oxide-coated sand. ASCE J. Environ.Engineering. 122(8), 769 (1996), which discloses several methods for theremoval of arsenic from contaminated water to the consumable limit.

In U.S. Pat. No. 4,566,975, heavy metals such as arsenic are removed ina two step process which involves an alkaline precipitation carried outat a pH of at least about 8 and using ferrous sulfate as an additive.

In U.S. Pat. No. 4,880,510, the electrolytic cell has been used toremove color impurities such as dyes from wastewater solution. Theferrous iron generated at the anode reacts with hydroxide ion to form aniron complex or compound, which further was found to react with orotherwise remove the color bodies from aqueous media as an insolubleprecipitate.

In U.S. Pat. No. 4,490,257, contaminants are removed by electrolysisprocess. The electrodes are resistant to corrosion.

In U.S. Pat. No. 5,043,080, contaminated groundwater is treated withhydrogen peroxide and transition metal ions at an acid pH in thepresence of ultraviolet light. The main object, however, is the removalof organic contaminants rather than heavy metals,

In U.S. Pat. No. 4,163,716, it was recognized that heavy metals andcolor bodies from dye house affluent could be removed with ferrous ionssupplied by iron electrodes with the ferrous ion oxidizing to the ferricstate by use of an oxidizing agent such as hydrogen peroxide. At a pH ofbetween 7 and 9, heavy metals and traces of color adhere to the ferrichydroxide floc, which then may be removed. This process also involves apH adjustment from a reaction pH of below 6.5 to a pH of from 7 to 9 toachieve removal of color particles.

The common technologies used for removal of arsenic are oxidation,co-precipitation, adsorption onto sorptive media, ion exchange resin andmembrane techniques etc. Presently, various materials like activatedcarbon (AC), zirconium coated activated carbon (Zr-AC) [B. Daus, R.Wennrich and H. Weiss, Sorption materials for arsenic removal fromwater: a comparative study. Water Res. 38(12), 2948 (2004)]; ironhydroxide [W. Wang, D. Bejan and N. J. Bunco, Removal of arsenic fromsynthetic acid mine drainage by electrochemical pH adjustment andco-precipitation with iron hydroxide. Environ. Sci. Technol. 37(19),4500 (2003)]; iron (II) and iron (III) oxides [L. C. Roberts, S. J. Hug,T. Ruettimann, M. Billah, A. W. Khan and M. T. Rahman; Arsenic removalwith iron (II) and iron (III) in waters with high silicate and phosphateconcentrations, Environ. Sci. Technol. 38(1), 307 (2004)], sand andzero-valent iron [O. X. Leupin and S. J. Hug. Oxidation and removal ofarsenic(III) from aerated groundwater by filtration through sand andzero-valent iron; Water Res. 39(9), 1729 (2005)], hardened paste ofPortland cement [Kundu, S. S. Kavalakatt, A. Pal, S. K. Ghosh, M. Mandaland T. Pal; Removal of arsenic using hardened paste of Portland cement:batch adsorption and column study. Water Res. 38(17), 3780 (2004)]: ironoxide coated polymers [A. Katasoyiannis and A. I. Zouboulis, Removal ofarsenic from contaminated water sources by sorption ontoiron-oxide-coated polymeric materials, Water Res. 36(20), 5141 (2002)];biological systems (bacteria) [A. Katasoyiannis and A. I. Zouboulis;[Application of biological processes for the removal of arsenic fromgroundwater] Water Res. 38(1), 17 (2004)] has been removed arsenic fromcontaminated water through biological process and could be used fordrinking and other household utilities.

The above materials and methods are effective and reduce arsenicconcentration in the potable water up to acceptable limits. But thesematerials and methods have their own advantages and disadvantages like,oxidation process is very simple and low cost but it is very slow andremoves only a part of the arsenic, co-precipitation by alum or iron isagain simple and low capital arrangement but it produces toxic sludge'sand pre-oxidation is required to start the reaction.

The use of iron or iron oxide for removing arsenic is dominative as itis very cheap, highly effective and can purify large volume of water. Inthis process, arsenite (As³⁺) species is first oxidized to arsenate(As⁵⁺) in the presence of atmospheric oxygen, or Ozone or free chlorine.

Reference may be made to G. Hering, P. Y. Chen. J. A. Wilkie, M.Elimelech and S. Liang. Arsenic removal by ferric chloride. J. AmericanWater Works Association; 88(4), 155 (1996). Wherein the arsenate speciesgot adsorbed over the surface of iron oxide during filtration and areremoved from the contaminated water. Roberts L. C., et al. have used Fe(II) and Fe (III) to remove arsenic from water with high silicate andphosphate concentrations.

Reference may be made to Daus B. et al. Water Res. 2004 Jul; 38-(12):2948-54 that has proved that arsenite and arsenate can be removed fromwater using Activated carbon (AC), zirconium loaded Activated carbonwith other materials successfully.

Reference may be made to Water Res. 2003 May; 37(10): 2478-88, whereinthe arsenic was effectively removed by steel manufacturing byproductslike evaporation cooler dust (ECD), oxygen gas sludge (OGS), and basicoxygen furnace slag (BOFS).

Reference may be made to Bang S. et al. J. Hazard Mater. 2005 May 20,121(1-3): 61-67, who has reported effect of dissolved oxygen and pH onthe removal of Arsenic from water and concluded that at pH 6 thatarsenate removal (99.8%) was faster than arsenite (82.6%) and moredissolved oxygen and low pH increases the rate of iron corrosion andleads to the formation of iron hydroxide, which ultimately adsorbsarsenic from the solution.

Reference may be made to S. Kundu, S. S. Kavalakatt, A. Pal, S. K.Ghosh, M. Mandal, and I. Pal, Water Res. 2004 Oct; 38(17): 3780-90wherein Portland cement (HPPC) paste has been used as adsorbent for theremoval of arsenic from water and have shown that 95% arsenate and 88%Arsenite can be removed easily.

Reference may be made to Sarkar, A. et al. Water Res. 2005 May; 39(10):2196-206 wherein activated alumina has been used as adsorbent forarsenic removal from drinking water.

Reference may be made to Bang S. et al. Chemosphere. (2005) Jul; 60(3):389-97 wherein granular titanium dioxide (TiO₂) has been used for theremoval of arsenic from groundwater.

Reference may be made to Oklahoma State University, USA, Advancedceramic reports; Issue: August 2004, page: 6 wherein Porous Zinc oxideheads has been used to remove arsenic from the contaminated water.

Chromium is a common heavy metal contaminant of water supplies, largelyarising from the textile, leather and wood production industries. Themetal industry mainly discharged trivalent chromium. Hexavalent chromiumin industrial wastewater mainly originates from tanning and painting.Chromium may be applied as a catalyser, in wood impregnation, in audioand video production and in lasers. Chromite is the starting product forinflammable material and chemical production. Levels of chromium indrinking water have been controlled in the past by expensive, oftentoxic chemical based cleansing procedures.

Trivalent chromium is a dietary requirement for a number of organisms astrivalent chromium is an essential trace element for humans and withinsulin it removes glucose from blood and also plays a vital role in fatmetabolism.

But hexavalent chromium is very toxic to flora and fauna. The human bodycontains approximately 0.03 ppm of chromium. Daily intake of chromiumdepends upon feed and levels, and is usually approximately 15-200 μg,but may be as high as 1 mg. The Placenta is the organ having highestchromium amounts. Chromium deficits may enhance diabetes symptoms.Chromium can also be found in RNA. Chromium deficits are very rare, andchromium feed supplements is not often applied. Chromium (III) toxicityis unlikely, at least when it is taken up through food and drinkingwater. It may even improve health, and cure neuropathy andencephalopathy. Hexavalent chromium is known for its negative health andenvironmental impact. It causes allergic and asthmatic reactions and itis 1000 times more toxic than trivalent chromium. Exposure to hexavalentchromium causes diarrhoea, stomach and intestinal bleeding, cramps,paralysis and liver and kidney damage. The hexavalent chromium ismutagenic and carcinogenic in nature. Toxic effects may be passed on tochildren through the placenta. Chromium oxide is a strong oxidant andafter dissolution it forms chromium acid, which corrodes the organs. Thelethal dose is approximately 1-2 gm. Most countries apply a legal limitof 50 ppb chromium in drinking water. A professional illness in chromiumindustries is chromium sores upon skin contact with chromates. Chromiumtrioxide dust uptake in the workplace may cause cancer, and damage therespiration tract.

Common Cr(VI) removal technologies for drinking water applications areion exchange, membranes, reduction/precipitation/coagulation/filtration,sorptive media etc. The trivalent chromium can be removed by contactingthe solution with a weak acid cation exchange resin. The chromate can beremoved by a weak base anion exchange resin in the presence of acid[Chopra, Randhir C, “Removal of chromium, chromate, molybdate and zinc”U.S. Pat. No. 3,972,810 (1976)]. Each of the resins requires differentregenerate so that the process will require bulky equipment due to therequirement for separate sites for the regeneration of the two resins.Thus the chromium can be removed but the other pollutant is added to itssolution. The most common industrial chromium treatment methods arereduction/precipitation/filtration. In this process, the Cr(VI) isreduced to Cr(III) typically by some reductant and chromium precipitatedout as Cr(OH)₃ and further coagulation were carried out with ferric saltand filtered. [Besselievre, E. B. (1969); The treatment of industrialwastes, McGraw-Hill, New York].

Reference may be made to Several U.S. Pat. Nos. 3,926,754; 4,036,726;and 4,123,339, which claims removal of hexa or trivalent chromium fromwastewater electrochemically. In these patents, a process is describedwherein wastewater containing hexavalent chromium ions is caused to flowbetween a plurality of electrodes. When the anode has a surface ofiron/iron alloy/insoluble iron compound, an iron hydroxide derivativewill be produced electrochemically. In this process, hexavalent chromiumundergoes cathodic reduction to form trivalent chromium as insolublechromic hydroxide, which complexes with iron at the anode The trivalentchromium compound, physically or chemically combine with the insolubleiron derivative to thereby permit removal from solution. The precipitateis then removed from aqueous by any conventional techniques.

Reference may be made to Jakobsen, K. and Laska, R. (1977) Advancedtreatment methods for electroplating wasters, Pollution Engineering,8:42-46] wherein aspearin as resin have been used in ion exchange forremoval of chromium.

Another way of removing Cr(VI) from drinking water is to reduce theCr(VI) to Cr(III) and precipitate it as chromium hydroxide. Referencemay be made to El-Shafey. J. Phys. IV France, 107(2003) 419 whereincarbon sorbent has been used to remove Cr (VI) from aqueous solutions inthe pH range 2.2-2.6.

Reference may be made to A. Li Bojic, M Purenovic and D Bojic, Water SA, 30(3), 2004, wherein micro-alloyed aluminium composite (MAIC) hasbeen used as reducing agent for Cr(VI) removal from water. The mechanismof action is based on processes of reduction and co-precipitation byAl(OH)₃, because Cr(VI) is removed from the water phase as metalchromium and insoluble Cr(OH)₃

Reference may be made to Liora Rosenthal-Toib et al., Synthesis ofstabilized nanoparticles of zinc peroxide, Chemical Engineering JournalVol 136, March 2008, wherein stabilized nanoparticles of zinc peroxidewere prepared by an oxidation-hydrolysis-precipitation procedure.However the surface modifiers used in the present invention aredifferent from the one reported in prior art.

OBJECTIVE OF THE INVENTION

The main objective of the present invention is to provide a process forthe removal of arsenic and Cr(III&VI) from the contaminated water.

Another objective of the present invention is to reduce the size of thezinc peroxide nanoparticles suitable for the removal of arsenic andCr(III&VI) from the contaminated water

Another object of the present invention is to provide a low cost, highyield and simple process for the removal of arsenic and Cr(III&VI) fromthe contaminated water by using nanoparticles of zinc peroxide.

Yet another object is to provide a process for the removal of arsenicand Cr(III&VI) from the contaminated water without getting any change inthe pH of water after treatment.

Yet another object is to provide low cost ZnO, nanoparticles capped withglycerol/PVP/TEA having size 20±5 nm for the removal of arsenic andchromium from the contaminated water.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a process for the removal ofarsenic and chromium from contaminated water using zinc peroxidenanoparticles comprising treating the contaminated water containingarsenic and chromium with the nanoparticles of zinc peroxide in a ratio(w/v) ranging from 8:1 to 12:1 (mg/ml), having the concentration ofarsenic, Cr(III&VI) contamination below 50 ppm in water, at atemperature of 25-30° C., for a period of 5-10 min, followed byfiltration to obtain the desired low concentrated contaminationpermissible drinking water.

In an embodiment of the present invention, the zinc peroxidenanoparticles used have size below 50 nm.

In another embodiment of the present invention, the amount of zincperoxide used for the removal of arsenic and chromium is preferably inthe range of 225-250 mg per 25 ppm of arsenic and chromium contaminatedwater.

In another embodiment of the present invention, the zinc peroxidenanoparticles are prepared by the process comprising the followingsteps:

-   a) dissolving zinc acetate in ammonium hydroxide to obtain a    solution mixture having pH in the range of 9-11;-   b) adding glycerol/PVP/TEA to the solution mixture obtained in    step (a) in a ratio of zinc acetate to glycerol in the range of    2:0.5 to 6:1 (w/w), PVP in the range of 10:0.25 to 10:1 (w/w) and    TEA in the range of 1:1 to 1:0.5 (w/w) with respect to zinc acetate    respectively;-   c) adding a polar organic solvent to the solution mixture obtained    in step (h), under stirring, at ambient temperature in the range of    25-30° C., followed by adding equimolar quantity of hydrogen    peroxide with respect to zinc acetate to obtain the desired    nanoparticles of zinc peroxide.

In another embodiment of the present invention, the weight ratio of zincacetate to glycerol used is preferably in the range of 2:0.5 to 3:1.

In another embodiment of the present invention, the weight ratio of zincacetate to PVP used is preferably in the range of 10:0.5 to 10:1 (w/w).

In another embodiment of the present invention, the weight ratio of zincacetate to TEA used is preferably in the range of 1:1 to 1:0.5 (w/w).

In another embodiment of the present invention, yield of ZnO₂nanoparticles obtained is in the range of 98-99% with respect to thestarting material.

In another embodiment of the present invention, the ZnO₂ nanoparticlesremoves arsenic, Cr(III&VI) each from 50 ppm to 0.1, 0.14, and 0.18 ppm,respectively without disturbing the pH of water.

In another embodiment of the present invention, the ZnO₂ nanoparticlesremoves arsenic, Cr(III&VI) each from 30 ppm to 0.03, 0.07 and 0.08 ppm,respectively without disturbing pH of the water.

In yet another embodiment of the present invention, the ZnO₂nanoparticles removes arsenic, Cr(III&VI) each from 25 ppm to less thanthe detection limit of AAS-HG in case of arsenic and 0.01 in case ofCr(III&VI) without disturbing pH of the water.

In yet another embodiment of the present invention, the ZnO₂nanoparticles removes arsenic, Cr(III&VI) each from 20 ppm to less thanthe detection limit of AAS-HG and FAAS/GFAAS respectively withoutdisturbing pH of the water.

In yet another embodiment of the present invention, the ZnO₂nanoparticles removes arsenic, Cr(III&VI) each from 15 ppm to less thanthe detection limit of AAS-HG and FAAS/GFAAS respectively withoutdisturbing pH of the water.

In yet another embodiment of the present invention, the ZnO₂nanoparticles removes arsenic, Cr(III&VI) each from 10 ppm to less thanthe detection limit of AAS-HG and FAAS/GFAAS respectively withoutdisturbing pH of the water.

In still another embodiment of the present invention, the ZnO₂nanoparticles removes arsenic, Cr(III&VI) each from 5 ppm to less thanthe detection limit of AAS-HG and FAAS/GFAAS respectively withoutdisturbing pH of the water

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 XRD Pattern of ZnO₂ powder

FIG. 2 TEM of ZnO₂ powder as synthesized

FIG. 3 TEM and EDS of ZnO₂ powder after reacting with arsenic (showingadsorption of arsenic on the surface of zinc peroxide)

FIG. 4 TEM of ZnO₂ powder after removing Arsenic

FIG. 5 TEM and EDS of ZnO₂ powder after reacting with chromium (showingadsorption of chromium on the surface of zinc peroxide)

FIG. 6 TEM of ZnO₂ powder after removing chromium

FIG. 7 FTIR of ZnO₂ powder after arsenic removing

FIG. 8 FTIR of ZnO₂ powder after Cr(III&VI) removal

DETAILED DESCRIPTION OF THE INVENTION

The present invention demonstrates the use of chemically prepared ZnO₂nanoparticles capped with glycerol and PVP for removal of arsenic andchromium metal ion from the contaminated water. The synthesized ZnO₂nanoparticles remove arsenic and chromium from the contaminated waterfrom the level of 25 ppm to less than the detection limits of the AAS-HGand FAAS/GFAAS instruments used respectively, which is less than theUSEPA prescribed limit 10 ppb for arsenic and 50 ppb for chromium fordrinking water.

Synthesis of Zinc Peroxide Nanoparticles

In the present invention ZnO₂ nanoparticles, used for arsenic andchromium removal, were synthesized using zinc acetate di-hydrate asprecursors in ammonical water medium at room temperature. For thesynthesis of ZnO₂, 10 gm of zinc acetate was dissolved in minimumquantity of ammonia solution and it was diluted to 200 ml byacetone/methanol/ethanol water mixture (Water: solvent: 4:1). 0.5 gm PVPor 2.5 gm of glycerol or 1:1 quantity of TEA of with respect to zincacetate were added to reduce the size of particles of the zinc peroxide.Further, 65 ml of hydrogen peroxide was added in above solution at pH9-11 at room temperature. The maximum quantity of zinc peroxidenanoparticles of 20±5 nm were synthesized by varying quantity of thecapping agent, pH and by varying solvent to water ratio. The solutionwas stirred on magnetic stirrer for 1 hour after adding hydrogenperoxide solution. The precipitate was centrifuged washed several timeswith 1:1 water-solvent mixture and then with de-ionized water severaltimes. Finally the precipitate was dried at 105° C. in an oven up tocomplete dryness. XRD of synthesized zinc peroxide nanoparticles showspure phase of zinc peroxide and HRTEM micrograph shows spherical natureof the nanoparticles. We have also synthesized particles of sizes 10±5and 45±5 by varying the solvent and capping agent concentration. Theefficiency increases as the particles sizes reduces. But at the sametime it is not possible for us to separate the nanoparticles of zincperoxide without centrifuge because particles of lower sizes float inthe water and without centrifuge does not settled down. The particles ofbigger sizes as claimed gets settled down and water can be separateeasily. However we are in the process to make some device, which canfilter the particles of low size.

Arsenic/Chromium Metal Ion Removal Using ZnO₂ Nanoparticles:

Standard arsenic and Cr(III) solutions of 1000-ppm concentration (SCPscience USA) were used after desired dilutions, while 1000-ppm of Cr(VI)was prepared from high purity potassium dichromate following primarymethod and it was further diluted to desired range by proper dilution.

For each experiment known quantity (by weight) of ZnO₂ nanoparticlespowder was mixed in 25 ml of standard arsenic/Cr(III) and Cr(VI)solution in an ultrasonic cleaner (5-10 min). The mixture was thensubjected to 2-3minutes of centrifugation process to remove the ZnO₂nanoparticles. The remaining solution was filtered by any known methodto remove the residual ZnO₂ nanoparticles like Buckner funnel. Thefiltered water was then tested for the left over arsenic, Cr(III) andCr(VI) content.

In the present invention a highly sensitive hydride generation atomicabsorption spectroscopy (HGAAS) was used in which arsenic in presence ofhydrochloric acid and sodium borohydride forms arsenic hydride (AsH₃),which atomizes at 900° C. and produces the spectra. This technique ishighly sensitive and arsenic can be detected up to 0.03 ppb.

The determination of Cr(III&VI) was carried out by Flame atomicabsorption spectrometer at the optimize conditions for chromium, whilethe lower concentration less than 0.1 ppm were analyzed by graphitefurnace atomic absorption spectrometer.

The novelty of the invention lies in the use of ZnO₂ nanoparticles asthe adsorbent for arsenic/Cr(III&VI) metal ions and the level up towhich it removes the arsenic/Cr(III&VI) concentration from 25 ppm to thepotable limits (less than 0.01 ppm in case of arsenic and 0.01 n case ofCr(III&VI)).

Several experiment were carried out for varying quantity of arsenic.Cr(III&VI) with fixed quantity of ZnO₂ nanoparticles.

The following examples are given by the way of illustration andtherefore should not be construed to limit the scope of the invention.

EXAMPLE 1 Methods for the Preparation of ZnO₂ Nanoparticles UsingGlycerol as Surface Modifier

-   (i) 10 gm of zinc acetate was dissolved in 15 mL of ammonia solution    and it was diluted to 200 ml of aqueous acetone (Water to solvent:    4:1). 2.5 gm of glycerol was added to the above solution mixture at    pH 9.5 followed by adding 65 ml of hydrogen peroxide to obtain the    nanoparticles of the zinc peroxide having average particle size    distribution of 20±5 nm.-   (ii) 10 gm of zinc acetate was dissolved in 15 mL of ammonia    solution and it was diluted to 200 ml of aqueous acetone (Water to    solvent: 4:1). 5 gm of glycerol was added to the above solution    mixture at pH 9.5 followed by adding 65 ml of hydrogen peroxide to    obtain the nanoparticles of the zinc peroxide having average    particle size distribution of 10±5 nm.

(iii) 10 gm of zinc acetate was dissolved in 15 ml, of ammonia solutionand it was diluted to 200 ml of aqueous acetone (Water to solvent: 4:1).0.5 gm of glycerol was added to the above solution mixture at pH 9.5followed by adding 65 ml of hydrogen peroxide to obtain thenanoparticles of the zinc peroxide having average particle sizedistribution of 45±5 nm.

EXAMPLE 2 Preparation of ZnO₂ Nanoparticles Using PVP as SurfaceModifier

(i) 10 gm of zinc acetate was dissolved in 15 ml, of ammonia solutionand it was diluted to 200 ml of aqueous methanol (Water to solvent:4:1). 0.5 gm of PVP was added to the above solution mixture at pH 10,followed by adding 65 ml of hydrogen peroxide to obtain thenanoparticles of the zinc peroxide having average particle sizedistribution of 20±5 nm.

-   (ii) 10 gm of zinc acetate was dissolved in 15 mL of ammonia    solution and it was diluted to 200 ml of aqueous methanol (Water to    solvent: 4:1). 1 gm of PVP was added to the above solution mixture    at pH 10, followed by adding 65 ml of hydrogen peroxide to obtain    the nanoparticles of the zinc peroxide having average particle size    distribution of 10±5 nm.-   (iii) 10 gm of zinc acetate was dissolved in 15 mL of ammonia    solution and it was diluted to 200 ml of aqueous methanol (Water to    solvent: 4:1). 0.15 gm of PVP was added to the above solution    mixture at pH 10, followed by adding 65 ml of hydrogen peroxide to    obtain the nanoparticles of the zinc peroxide having average    particle size distribution of 45+5 nm.

EXAMPLE 3 Preparation of ZnO₂ Nanoparticles Using TEA as SurfaceModifier

-   (i) 10 gm of zinc acetate was dissolved in 15 mL of ammonia solution    and it was diluted to 200 ml of aqueous ethanol (Water to solvent:    4:1). 5 gm of TEA was added to the above solution mixture at pH 11,    followed by adding 65 ml of hydrogen peroxide to obtain the    nanoparticles of the zinc peroxide having average particle size    distribution of 20±5 nm.-   (ii) 10 gm of zinc acetate was dissolved in 15 mL of ammonia    solution and it was diluted to 200 ml of aqueous ethanol (Water to    solvent: 4:1). 10 gm of TEA was added to the above solution mixture    at pH 11. followed by adding 65 ml of hydrogen peroxide to obtain    the nanoparticles of the zinc peroxide having average particle size    distribution of 10±15 nm.-   (iii) 10 gm of zinc acetate was dissolved in 15 ml, of ammonia    solution and it was diluted to 200 ml of aqueous ethanol (Water to    solvent: 4:1). 1.5 gm of TEA was added to the above solution mixture    at pH 11, followed by adding 65 ml of hydrogen peroxide to obtain    the nanoparticles of the zinc peroxide having average particle size    distribution of 45+5 nm.

EXAMPLE 4 Removal of Arsenic and Chromium From Water Using Zinc PeroxideNanoparticles

Three different ranges nonmaterial i.e. 10+5, 45+5 and 20±5 nm sizeshave been used for the removal of arsenic and Cr(III&VI). All the threeranges of ZnO₂ nanoparticles remove arsenic and Cr(III&VI) effectively.The particles of 10±5 nm sizes requires centrifuge after process toseparate out from the water because the small particles get filteredthrough Buckner funnel. The arsenic and Cr(III&VI) efficiency ofnanoparticles reduces as the size of nanoparticles increases (>50 nm).The following example has been given for the 20+5 nm size nanoparticles.

Quantity of ZnO₂ having average particle size distribution of 20+5 nmtaken for process=0.25 g

Water taken=25 ml

Concentration of Arsenic and Cr(III&VI) taken=50 ppm

Concentration of Arsenic after treatment with ZnO₂=0.1 ppm

Concentration of Cr(III) after treatment with ZnO₂=0.14 ppm

Concentration of Cr(VI) after treatment with ZnO₂=0.18 ppm

EXAMPLE 5

Quantity of ZnO₂ having average particle size distribution of 20±5 nmtaken for process=0.25 g

Water taken=25 ml

Concentration of Arsenic and Cr(III&VI) taken=30 ppm

Concentration of Arsenic after treatment with ZnO₂=0.03 ppm

Concentration of Cr(III) after treatment with ZnO₂=0.07 ppm

Concentration of Cr(VI) after treatment with ZnO₂=0.08 ppm

EXAMPLE 6

Quantity of ZnO₂ having average particle size distribution of 20±5 nmtaken for process=0.25 g

Water taken=25 ml

Concentration of Arsenic and Cr(III&VI) taken=25 ppm

Concentration of Arsenic after treatment with ZnO₂=below detection limitof AAS-HG instrument

Concentration of Cr(III) after treatment with ZnO₂=less than 0.01 ppm byGFAAS instrument

Concentration of Cr(VI) after treatment with ZnO₂=0.01 ppm by GFAASinstrument

EXAMPLE 7

Quantity of ZnO₂ having average particle size distribution of 20±5 nmtaken for process=0.25 g

Water taken=25 ml

Concentration of Arsenic and Cr(III&VI) taken=20 ppm

Concentration of Arsenic after treatment with ZnO₂=below detection limitof AAS-HG instrument

Concentration of Cr(III) after treatment with ZnO²=below detection limitof FAAS/GFAAS instrument

Concentration of Cr(VI) after treatment with ZnO₂=below detection limitof FAAS/GFAAS instrument

EXAMPLE 8

Quantity of ZnO₂ having average particle size distribution of 20±5 nmtaken for process=0.25 g

Water taken=25 ml

Concentration of Arsenic and Cr(III&VI) taken=15 ppm

Concentration of Arsenic after treatment with ZnO₂=below detection limitof AAS-HG instrument

Concentration of Cr(III) after treatment with ZnO₂=below detection limitFAAS/GFAAS of instrument

Concentration of Cr(VI) after treatment with ZnO₂=below detection limitFAAS/GFAAS of instrument

EXAMPLE 9

Quantity of ZnO₂ having average particle size distribution of 20±5 nmtaken for process=0.25 g

Water taken=25 ml

Concentration of Arsenic and Cr(III&VI) taken=10 ppm

Concentration of Arsenic after treatment with ZnO₂=below detection limitof AAS-HG instrument

Concentration of Cr(III) after treatment with ZnO₂=below detection limitFAAS/GFAAS of instrument

Concentration of Cr(VI) after treatment with ZnO₂=below detection limitFAAS/GFAAS of instrument

EXAMPLE 10

Quantity of ZnO₂ having average particle size distribution of 20±5 nmtaken for process=0.25 g

Water taken=25 ml

Concentration of Arsenic, Cr(III&VI) taken=5 ppm

Concentration of Arsenic after treatment with ZnO₂=below detection limitof AAS-HG instrument

Concentration of Cr(III) after treatment with ZnO₂=below detection limitFAAS/GFAAS of instrument

Concentration of Cr(VI) after treatment with ZnO₂=below detection limitFAAS/GFAAS of instrument

Advantages of the Present Invention:

-   1. This invention makes the contaminated water almost free of    arsenic and Cr(III&VI) after filtration that could be use for    drinking and various household utilities.-   2. Treatment with ZnO₂ nanoparticles maintains the pH of the water    to the permissible limit.-   3. The proposed method for the synthesis of zinc peroxide is    eco-friendly and there is no generation of any toxic gases during    synthesis.-   4. The material was synthesized at low temperature and there is no    requirement of any specific instruments.-   5. The method gives more than 98% yield, which is an added advantage    of the process.-   6. The proposed material is low cost in comparison to other    materials available for the removal of arsenic.-   7. The solvent used in the synthesis process can be recovered by    distillation, which is further added advantages of the process for    reduction of the cost of the process.-   8. The process does not add any impurities in the water as it is    insoluble in water and can be separated by any known method.-   9. The synthesis method is so simple and fast and within 2-3 hours    on can get synthesized nanoparticles of zinc peroxide with more than    98% yield.-   10. The particles of 10+5 are more efficiently removes arsenic and    Cr(III&VI) from contaminated water but in this process    centrifugation is essential to separate zinc peroxide from the    water. The particles of 10±5 nm sizes can be synthesized by varying    the concentration of solvent and capping agent.

1-16. (canceled)
 17. A process for preparing zinc peroxide nanoparticlescomprising the following steps: a) dissolving zinc acetate in ammoniumhydroxide to obtain a solution mixture having pH in the range of 9-11;b) adding glycerol/PVP/TEA to the solution mixture obtained in step (a)in a ratio of zinc acetate to glycerol in the range of 2:0.5 to 6:1(w/w), PVP in the range of 10:0.25 to 10:1 (w/w) and TEA in the range of1:1 to 1:0.5 (w/w) with respect to zinc acetate respectively; c) addinga polar organic solvent to the solution Mixture obtained in step (b),under stirring, at ambient temperature in the range of 25-30° C.,followed by adding equimolar quantity of hydrogen peroxide with respectto zinc acetate to obtain the desired nanoparticles of zinc peroxide.18. The process as claimed in claim 17, wherein the weight ratio of zincacetate to glycerol used is in the range of 2:0.5 to 3:1.
 19. Theprocess as claimed in claim 17, wherein the weight ratio of zinc acetateto PVP used is in the range of 10:0.5 to 10:1 (w/w).
 20. The process asclaimed in claim 17, wherein the weight ratio of zinc acetate to TEAused is in the range of 1:1 to 1:0.5 (w/w).
 21. The process as claimedin claim 17, wherein yield of ZnO₂ nanoparticles obtained is in therange of 98-99% with respect to the starting material.
 22. The processas claimed in claim 17, wherein the size of zinc peroxide nanoparticlesis below 50 nm.
 23. The process as claimed in claim 22, wherein the sizeof zinc peroxide nanoparticles is 20±5 nm.